Heretofore, it has been proposed to use polymeric plastic materials to enhance the general behavior of cement-based parts, by incorporating such materials into the cement mix by means of varied techniques. According to one approach, for example, liquid polymeric-resin systems have been formulated to be embodied into mortars and concretes, developing thereby in their end products improved tensile strength and compressive strength, as well as characteristics of chemical and abrassion-resistance. Thus, a polymer mortar is prepared by partially replacing cement with polymer in the respective cementitious mix. This process requires the use on the one hand, of specific liquid polymer systems (resins, accelerators and catalysts) and, on the other, apparatus specially designed for proportioning and blending the appropriate amounts of materials. To that effect, the plastic industry has devised suitable polymer systems comprising resin as their main constituent, where the particular resin has often been selected from polyester and methyl- or polymethyl-methacrylate resins, together with promoters and frequently other proprietary additives. As a result, cement-mix processors depend on the use of hardly available materials. Moreover, another limitation which discourages the widespread use of the polymer resin-cement techniques is brought about by the need to use sophisticated and expensive proportioning and blending machines.
Also, in the always continuing efforts to enhance properties of parts manufactured from cement mixtures, particularly to increase the strength of cement-based building elements subjected to impact loading, the use of plastic filamentary reinforcing means has been proposed. Plastic fibers, however, have proved to be unsuitable in some respects as reinforcing materials. A main drawback is the one arising in great measure from the plastic fibers' hydrophobic nature, which comes to impair their frictional adhesion with the cementitious matrix whereby the stressed plastic filaments become debonded. The problem brought about by the poor physic-chemical adherence with their matrix is also involved to deter further the use of the so far commercially available plastic fibers as reinforcing means for cement products. In order to overcome such a hindrance posed to the use of monofilamentary plastic fibers it has been proposed to use certain plastic materials in the form of fibrillated films, as a reinforcing means for cement-based elements. The foregoing approach has permitted indeed the succesful attain went of good mechanical bonding of the reinforcing fibrillated films with the cement matrix.
Typical of the foregoing technique of manufacturing cement-based parts reinforced with plastic fibers is the U.K. Pat. No. 1,582,945 issued on Jan. 21, 1981, to the University of Surrey and D. J. Hannant. The patent teaches, for reinforcing purposes to use fibrillated polypropylene film, left as a structure comprised of spread-out, non-woven elongated fibers forming a film-like mesh, or used as a woven-mesh structure. In either case, the films are given a open-up, continuous structure having its dimensions and shape such as to closely fit the dimensions of the sheet-like, plastic-reinforced article. The plastic reinforced sheets manufctured by the method of the patent are necessarily layered structures, having the fibers as a single-direction reinforcement, as a direct consequence of their manufacturing process. The sheets have been proposed as suitable substitutes for asbestos-reinforced cement articles. However, certain conditions, particularly under high temperature conditions bring about spallation and delamination of the aforecited layered structures.
The U.K. Pat. No. 1,586,512 issued on Mar. 18, 1981, to Dansk Eternit-Fabrik, Denmark, deals with the manufacturing of building of sheets wherein the cement is reinforced by means of polypropylene fibers; the cementitious mix is reinforced further with mineral flakes and other fibers; and the fibers are concentrated in layers in the sheets.
According to the technologies based on the use of fibrillated plastic films as the reinforcing means for cement-based parts, they are coated with cement mortar and the excess mortar removed from the coated films; the so coated layers are laid in sequence each coextensively arranged onto the other until piling up the required number of layers to make up the desired sheet thickness. The so prepared sheet while still in its fresh condition may be handled to be molded into the shape of the particular mold used, suited to the end product. The articles manufactured in that way, because of prevailing orientation of the fibrils in their reinforcing films in one direction only, have values for its mechanical properties greatly differing according to the directions in which they are oriented; that is, the articles are said to be remarkably anisotropic. The anisotropic nature is controlled then, in accordance to the orientation in which the plastic mesh is placed and according to the mesh opening.
As stated above, in accordance with the foregoing process the reinforcing plastic films are mechanically slit into a predetermined degree of fibrillation which will be preserved throughout the manufacturing process and it will be the same amount of fibrillation that will be present in the hardened cement product. Since the fibrillated film is embedded into a previously prepared mortar paste, and inasmuch as the plastic material is chosen from polyolefines, particularly polypropylene, which are hardly wettable materials, the process therefore does not provide for means to help promote the desirable humidification to develop appropriate adherence between the fibrous material and the matrix encapsulating it. Poor adhesion impairs the tensile strength of the cement mortar elements obtained by this process, as well as its ability to develop closely spaced multicracking with desirable small cracks. The development of thin cracks in a closely arranged pattern is a phenomenon normally occurring in hardened cement elements which have been reinforced by means of aligned polypropylene fibers.
Summarizing, there are prior art processes designed for replacing asbestos fibers with polypropylene fibers as a reinforcing material for cement-based elements, particularly reinforced cement sheets, boards and the like, and the literature abounds in references to such techniques. Even though satisfactory plastic fibers-reinforced cement showing properties bearing comparison with those of asbestos-reinforced cements have been obtained through prior art processes, their use is restrained first, because they in any event originate a material suitable only for sheet products; the reinforcing material is not efficiently used thereby; the fiber-cement material is suited to be shaped by one definite molding operation only; and also because the end products obtained therefrom are remarkably anisotropic in nature.